Legally debatable its uses may be, it is reliable and the amount of music available on it is vast and far larger than any bona fide download site could be.
I would be very wary of downloading from 'legitimate' sites as I will not use them until they alter their extortionate pricing policy, which is criminal condsidering the overheads they DONT have to pay, or that they may be reselling old music at high prices.
Imho Limewire is the best filesharing program in terms of being spyware free, stable, user friendly and utilising the Gnutella network which is very extensive. But, whatever you do, don't download copyrighted .mp3 files primarily due to the illegality of the operation but also coz your toe nails will curl up and your hair will drop out!;-)